1. Field of the Invention
The present invention relates to a zoom lens and more particularly to a small-sized zoom lens suitable for a compact camera.
2. Related Background Art
In taking lenses for recent shutter-type compact cameras, zoom lenses become the mainstream.
A zoom lens formed of positive and negative lens groups (two lens groups) is simple in structure and has a first lens group with a positive refractive power and a second lens group with a negative refractive power. For example, as such a zoom lens, there is a zoom lens disclosed in Japanese Patent Laid-Open Application No. 2-73322 wherein the zoom ratio is 2 to 1 and the number of lenses is small. In this zoom lens, in order to secure a sufficient back focus in a wide-angle end and to correct the positive distortion preferably, the image principal point of a positive lens group is located on the image side beyond a lens surface of the positive lens group located closest to the image side.
As zoom lenses with great zoom ratio, e.g., a zoom lens formed of positive, negative, positive and negative lens groups (four lens group) and a zoom lens formed of positive, positive and negative lens groups (three lens group) are known in which the number of lens groups is increased and the respective lens groups are moved independently in a refractive power varying operation from a wide-angle end to a telescopic end.
The zoom lens formed of the positive, positive and negative lens groups (three lens group) has, in the following order from the object side, a first lens group with a positive refractive power, a second lens group with a positive refractive power and a third lens group with a negative refractive power. When varying a refractive power from a wide-angle end to a telescopic end, the distance of an air space between the second lens group and the third lens group is decreased and the distance of an air space between the first lens group and the second lens group is increased, thereby enabling great zoom ratio.
For example, in a zoom lens disclosed in Japanese Patent Laid-Open Application No. 2-73211, a second lens group has a 2a lens group with a negative refractive power disposed on the object side and a 2b lens group with a positive refractive power disposed on the image side. And, by securing the distance between the 2a lens group and the 2b lens group properly widely, the negative distortion is produced by the 2a lens group in a wide-angle end to correct the positive distortion preferably, making it possible to obtain a sufficient back focus to achieve a wide-angle end to realize great zoom ratio.
On the other hand, for example, in Japanese Patent Laid-Open Application No. 1-252915, a zoom lens (formed of positive, negative, positive and negative lens groups (four lens groups)) has, in the following order from the object side, a first lens group with a positive refractive power, a second lens group with a negative refractive power, a third lens group with a positive refractive power and a fourth lens group with a negative refractive power. When varying a refractive power from a wide-angle end to a telescopic end, the distance between the first lens group and the second lens group is increased and the distance between the third lens group and the fourth lens group is decreased. Thereby, a change of the whole length of the zoom lens due to the refractive power varying operation is limited, making it possible to reduce the whole length of the zoom lens in the telescopic end as compared to the zoom lens formed of the positive and negative lens groups (two lens groups).
However, when achieving great zoom ratio in the zoom lens formed of the positive and negative lens groups (two groups), a change of the magnification of the negative lens group in the refractive power varying operation from the wide-angle end to the telescopic end becomes large. Therefore, it becomes difficult to preferably limit changes of various aberrations caused by the negative lens group during moving the lens groups.
Also, when achieving a wide-angle in the zoom lens formed of the positive and negative lens groups (two groups), a sufficient back focus cannot be obtained in the wide-angle end and effective diameters of rear lenses become large. Further, movement of the negative lens group is increased in the refractive power varying operation from the wide-angle end to the telescopic end. Also, since the image principal point of the positive lens group is located on the image side beyond the lens surface of the positive lens group located closest to the image side as mentioned above, the whole length of the zoom lens in the telescopic end becomes large, which is therefore insufficient in terms of miniaturization.
The zoom lens formed of the positive, positive and negative lens groups (three lens groups), in Japanese Patent Laid-Open Application No. 2-73211, is suitable for obtaining great zoom ratio as compared to the zoom lens formed of the positive and negative lens groups (two lens groups) . However, as mentioned above, since the 2a lens group has a diverging function and the distance between the 2a lens group and the 2b lens group is made large, the whole length of the zoom lens becomes large in the telescopic end as compared to the effective focal length, so it is insufficient to reduce the whole length of the zoom lens.
In the zoom lens formed of the positive, negative, positive and negative lens groups (four lens groups) in Japanese Patent Laid-Open Application No. 1-252915, as the distance of the air space between the second lens group and the third lens group is decreased in the refractive power varying operation from the wide-angle end to the telescopic end, zoom ratio larger than that of the zoom lens formed of the positive, positive and negative lens groups (three lens groups) can be obtained and the whole length of the zoom lens in the telescopic end can be reduced. However, since the refractive power of the second lens group is considerably large toward the negative side as compared to that of the fourth lens group, the reduction of the whole length of the zoom lens in the telescopic end is not sufficient. Also, as varying the refractive power from the wide-angle end to the telescopic end, principal rays passing the first lens group are spaced apart from the optical axis, so that the effective diameters of front lenses become large.